1. Field of the Invention
The present invention relates to non-contact type sensors and, more particularly, to a non-contact type sensor driven by a single power supply for measuring a voltage or current on a non-contact basis.
2. Description of the Related Art
Non-contact type sensors in the art to which the invention is related include those operating on a single power supply to measure a current or voltage. Such a non-contact type sensor has an amplifier for amplifying an output voltage of a Hall element provided in the gap at a magnetic core of a secondary coil. First and second windings are wound around the secondary coil in opposite directions. A current in a direction opposite to that of the current to be measured is applied to the first winding. A current of the same ampere turns as that of a bias current to the first winding is applied to the second winding through a variable resistor to maintain magnetic flux in the magnetic core of the secondary coil at zero. Thus, a current is applied so that a bias current for a secondary current is canceled to achieve magnetic equilibrium of the secondary coil, thereby obtaining a voltage proportionate to the current to be measured.
Non-contact sensors of the related art include another type proposed by the applicant in an unpublished application. This non-contact type sensor employs driving from a single power supply of +12 V and has a configuration wherein a current in a negative direction flows from a current buffer to a secondary winding and resistor to cancel a magnetic flux generated by a current flowing through a primary winding and, when a current in the opposite direction flows through the primary winding, a current in a positive direction flows through the resistor, secondary winding and current buffer. In such a non-contact type sensor of the related art, a mid-point is provided for charging and discharging an output current, and the level of the mid-point is changed by turning the current buffer on and off. When a single power supply of +12 V is employed and first and second resistors connected to each other in series in a constant voltage circuit have the same resistance, the voltage dividing point between the first and second resistors has a voltage of +6 V. A voltage follower connected to the first and second resistors outputs the voltage of +6 V at the voltage dividing mid-point to an output terminal. A detection resistor provided at the output terminal for voltage measurement has a voltage on the side of the secondary winding which is about 0 V in the positive direction of the current and about 12 V in the positive direction of the current. Therefore, the voltage across the resistor provided at the output terminal for voltage measurement is .+-.6 V and, the range of voltage measurement is +6 V.
Further, in this non-contact type sensor of the related art, the resistance of the first resistor in the constant voltage circuit is varied. For example, if the resistance of the first voltage is set at one-half of the resistance of the second resistor when a single power supply of +12 V is employed, the voltage follower has an output of (2/3) V, i.e., +8 V because it outputs the voltage at the voltage dividing point between the first and second resistors.
Then, a negative or positive current flows from the current buffer to the secondary winding, detection resistor and voltage follower or from the voltage follower to the detection resistor, secondary winding and current buffer. At this time, the voltage of the detection resistor on the side of the secondary winding is about 0 V in the positive direction of the current and about 12 V in the positive direction of the current. That is, the voltage across the detection resistor is 8 V in the positive direction and 4 V in the negative direction, and the ratio between measuring ranges in the positive and negative directions is 2:1. Thus, the measuring ranges in the positive and negative directions can be varied by changing the resistance of the first resistor.
However, when a subbattery of +12 V is used, the method described above wherein the level of the mid-point is varied provides a measuring range of .+-.6 V which is not wide enough.
There are non-contact type sensors which include a DC--DC converter and use power supplies of +12 V and -12 V to be capable of providing a measuring range of .+-.12 V which is wide enough. This results in increases in power consumption, in the capacity of transformers and in the size of the non-contact type sensors themselves.